1. Field of the Invention
The present invention relates to digital printing apparatus and methods, and more particularly to an apparatus for continuously supplying lithographic printing material to the plate cylinder of a planographic printing press or a plate-material imager.
2. Description of the Related Art
Traditional techniques of printing an image onto a recording medium, such as paper, include letterpress printing, gravure printing and offset lithography. All of these printing methods require the use of plate material. This plate material is loaded onto a rotating plate cylinder that is brought into pressurable contact with the recording/printing medium.
In letterpress printing, the image is represented on the plate material as raised surfaces that accept ink and transfer it onto the medium. Conversely, gravure plates define a series of wells into which the ink is deposited. Excess ink is removed from the plate material using a doctor blade or another closely contacting surface that strips excess ink from the plate material before it is brought into contact with the printing media, thereby transferring the ink to the medium.
In offset lithography, an image is defined on a printing plate defined by ink-accepting (oleophilic) areas surrounded by ink-repellent (oleophobic) surfaces. Two different lithographic systems are generally employed in offset lithography. In a dry printing system, the plate material is simply inked, and the image is transferred onto a recording/printing medium. First, the plate material makes contact with a compliant intermediate surface called a blanket cylinder which, in turn, applies the image to the paper or other medium. The paper is typically pinned to an impression cylinder in rolling contact with the blanket cylinder, which applies ink to the paper in accordance with the image.
In a wet lithographic system, the non-image areas are hydrophilic, and the necessary ink-repellency is provided by an initial application of a dampening (or "fountain") solution to the plate material prior to inking. The fountain solution prevents ink from adhering to the non-image areas but does not affect the oleophilic character of the image areas.
Many techniques have been developed for affixing plate material to underlying plate cylinders. Basic offset printing systems involve stationary clamping of a flexible length of plate material to the plate cylinder, while more advanced systems such as those described in U.S. Pat. Nos. 5,355,795 and 5,727,749 (both co-owned with the present application, and expressly incorporated herein by reference) use a relatively long length of plate material stored in the form of spools within a well in the plate cylinder. In these systems, a new segment of the plate material is advanced around the plate cylinder following completion of a print job. The new segment is imaged by an electronically controlled print head, which applies a print pattern to the surface.
It is important, during press operation, to maintain a substantial tension along the plate material that surrounds the plate cylinder. This material experiences significant tangential forces as a result of contact with the blanket cylinder, the forces resulting primarily from slight differences in the rolling diameters of the mating cylindrical surfaces, which are in contact at sufficient pressure to compress the compliant blanket cylinder surface. These forces will alter the orientation of the plate material or dislodge it completely unless the plate material is held with adequate tension against the plate cylinder. Accordingly, "payout" systems that dispense plate material from within the cylinder, must maintain strong contact between the plate material and the cylinder surface; at the same time, however, they must also allow sufficient relaxation to permit smooth supply and uptake of the material.
Typically, in order to maintain proper plate material tension during press operation, a mechanical tensioning mechanism is incorporated into the advancement system. That system includes a plate material supply spool and a take-up spool. As new plate material is needed, the take-up spool may be rotated under the action of a clutching motor while a lock (typically a ratchet and pawl assembly) is released on the supply spool, thereby allowing new plate material to be drawn therefrom. In particular, the '749 patent teaches a plate cylinder that regulates the tension of the plate material about the circumference of the cylinder. According to this patent, the plate material is advanced and tensioned by selectively locking and unlocking the supply spool while the clutching motor drives the take-up spool.
In accordance with the '749 patent, the supply spool is selectively unlocked by moving the pawl away from the ratchet to enable payout of new plate material by the supply spool. The take-up spool draws the used plate material around the cylinder until fresh material appears on the cylinder surface. The pawl is subsequently allowed to reengage the supply spool ratchet, thereby locking the supply spool to prevent further draw of plate material therefrom. The clutching motor, however, continues to drive the take-up spool, thereby creating a tension on the plate material about the circumference of the plate cylinder between the now-locked supply spool and the driven take-up spool. The tension is regulated by the preset braking torque of the clutch motor (i.e., the torque at which the clutch allows the motor to slip relative to the take-up spool drive shaft). The motor is shut off when it slips, and the thus-established tension is maintained by a one-way bearing on the take-up spool.
One potential disadvantage to the above-described system is that, during the plate-material advancement phase, tension tends to be concentrated at the take-up spool, particularly adjacent to the well in the cylinder within which the advancement mechanism resides. This results from inherent Coulomb friction generated by interaction between the back side of the plate material and the cylinder surface as the tension on the plate material is increased. It is found that this frictional resistance results even when the cylinder surface is highly polished. Indeed, the take-up spool may exert a tension roughly ten times that of the supply spool. One result of such tension is that it is difficult to control the plate material as it is advanced. In addition, the uneven tension generated adjacent the take-up spool can cause unwanted cylinder deflection. This tension imbalance also requires more powerful and, hence, larger motors and gearing for the advancement mechanism to establish a minimum tension across the entire exposed length of plate material. Another disadvantage to this type of system is the need for thick plate material capable of withstanding the tension generated by the Coulomb friction; increased thickness not only raises material costs, but limits the amount of plate material that can be accommodated within the plate cylinder.
One technique for reducing the friction (and the consequent tension in the plate is material as it is advanced around the plate cylinder) is to introduce additional pairs of supply and take-up spools. Multi-pair take-up and supply spools are described, for example, in U.S. Pat. No. 4,057,343 and in U.S. patent application Ser. No. 09/245,104 (the latter co-owned with the present application and expressly incorporated herein by reference). This approach reduces the length of plate cylinder surface over which each section of plate material must be advanced, which in turn reduces the accompanying tension in each section of plate material. Moreover, two segments of plate material are simultaneously available for imaging and printing from the same plate cylinder.
A disadvantage to this arrangement, however, is that it is mechanically complicated and expensive, requiring redundant mechanisms for proper plate material advancement. Another disadvantage is that precious space within the plate cylinder is lost in accommodating the duplicated components, resulting, once again, in a corresponding reduction in the amount of plate material that can be loaded into the plate cylinder.